This invention relates to a high pressure and high temperature digestion vessel assembly. The assembly is particularly adapted for use in a microwave oven and permits safe and effective chemical digestion for the purpose of chemical analysis. Microwave heating and digestion of organic samples in an enclosed high pressure and high temperature vessel has greatly shortened the amount of time required to perform these tests. The assembly solves several problems which have been observed in prior devices used for the same purpose. For example, U.S. Pat. No. 4,672,996 discloses a pressure vessel which includes a self-regulating valve positioned under a lid which is threaded onto the vessel. The valve includes a pressure-deformable, resilient wall member having a fluid vent port and an obstructing member which cooperates with the wall member to open the valve. While this device offers several advantages over prior devices, it nevertheless contains features which offer several problems.
The relief point of the valve is a function of the ability of the lid to deform and the torque with which the lid is tightened onto the vessel. Even with careful control of the torque, the deformation of the valve occurs at a temperature which cannot be accurately predicted. For this reason, both time and temperature remain variables since there is no direct temperature sensing within the vessel. Pressure resistance and distortion at various points on the vessel also vary.
The valve of the '996 Patent requires an expensive torque station to precisely tighten the lid on the vessel. The requirement of a torque station introduces a further variable, since it must be frequently calibrated and is subject to improper operation. Because of the design of the valve, vapors containing trace elements collect between the valve disk and the lid and condense when cooled. This liquid must be rinsed out with a known quantity of water and added back to the sample before the results of the digestion can be calculated. Even liquid trapped in the threads of the vessel can cause a variation in the results of the digestion.
The life of the device in the '996 Patent is limited to 30-50 tests, depending upon the pressure range of the test, since pressure tends to deform the vessel during each test. The pressure relief tube projects vertically out of the top of the vessel, meaning that the tube must be turned 180 degrees to carry any escaping vapor or liquid into a collection vessel. If the relief tube blows out of the valve, vapors may spray upwardly into the face of the operator.
Another prior art device is the Parr microwave acid digestion bomb. On occasion, the term "bomb" is quite apt. The hazards involved in using this product are recognized in literature distributed by the manufacturer, and include the possibility of uncontrolled explosion or discharge of contents under high temperature and pressure. Heating of the contents is solely by empirical determination of the proper combination of time and microwave power. In addition, a relatively expensive 0-ring must be replaced quite frequently.
The known prior art devices do not offer a combination of a safe digestion vessel with precise and repeatable temperature and pressure control.